275 research outputs found
Type Ia supernovae: explosions and progenitors
Supernovae are the brightest explosions in the universe. Supernovae in our Galaxy, rare and happening only every few centuries, have probably been observed since the beginnings of mankind. At first they were interpreted as religious omens but in the last half millennium they have increasingly been used to study the cosmos and our place in it. Tycho Brahe deduced from his observations of the famous supernova in 1572, that the stars, in contrast to the widely believe Aristotelian doctrine, were not immutable.
More than 400 years after Tycho made his paradigm changing discovery using SN 1572, and some 60 years after supernovae had been identified as distant dying stars, two teams changed the view of the world again using supernovae. The found that the Universe was accelerating in its expansion, a conclusion that could most easily be explained if more than 70% of the Universe was some previously un-identified form of matter now often referred to as `Dark Energy'.
Beyond their prominent role as tools to gauge our place in the Universe, supernovae themselves have been studied well over the past 75 years. We now know that there are two main physical causes of these cataclysmic events. One of these channels is the collapse of the core of a massive star. The observationally motivated classes Type II, Type Ib and Type Ic have been attributed to these events. This thesis, however is dedicated to the second group of supernovae, the thermonuclear explosions of degenerate carbon and oxygen rich material and lacking hydrogen - called Type Ia supernovae (SNe Ia). White dwarf stars are formed at the end of a typical star's life when nuclear burning ceases in the core, the outer envelope is ejected, with the degenerate core typically cooling for eternity. Theory predicts that such stars will self ignite when close to 1.38 Msun (called the Chandrasekhar Mass). Most stars however leave white dwarfs with 0.6 Msun, and no star leaves a remnant as heavy as 1.38 Msun, which suggests that they somehow need to acquire mass if they are to explode as SN Ia. Currently there are two major scenarios for this mass acquisition. In the favoured single degenerate scenario the white dwarf accretes matter from a companion star which is much younger in its evolutionary state. The less favoured double degenerate scenario sees the merger of two white dwarfs (with a total combined mass of more than 1.38 Msun).
This thesis has tried to answer the question about the mass acquisition in two ways. First the single degenerate scenario predicts a surviving companion post-explosion. We undertook an observational campaign to find this companion in two ancient supernovae (SN 1572 and SN 1006). Secondly, we have extended an existing code to extract the elemental and energy yields of SNe Ia spectra by automating spectra fitting to specific SNe Ia. This type of analysis, in turn, help diagnose to which of the two major progenitor scenarios is right
Helium in Double-Detonation Models of Type Ia Supernovae
The double-detonation explosion model has been considered a candidate for
explaining astrophysical transients with a wide range of luminosities. In this
model, a carbon-oxygen white dwarf star explodes following detonation of a
surface layer of helium. One potential signature of this explosion mechanism is
the presence of unburned helium in the outer ejecta, left over from the surface
helium layer. In this paper we present simple approximations to estimate the
optical depths of important He I lines in the ejecta of double-detonation
models. We use these approximations to compute synthetic spectra, including the
He I lines, for double-detonation models obtained from hydrodynamical explosion
simulations. Specifically, we focus on photospheric-phase predictions for the
near-infrared 10830 \AA~and 2 m lines of He I. We first consider a double
detonation model with a luminosity corresponding roughly to normal SNe Ia. This
model has a post-explosion unburned He mass of 0.03 and our
calculations suggest that the 2 m feature is expected to be very weak but
that the 10830 \AA~feature may have modest opacity in the outer ejecta.
Consequently, we suggest that a moderate-to-weak He I 10830 \AA~feature may be
expected to form in double-detonation explosions at epochs around maximum
light. However, the high velocities of unburned helium predicted by the model
(~km~s) mean that the He I 10830 \AA~feature may be
confused or blended with the C I 10690~\AA~line forming at lower velocities. We
also present calculations for the He I 10830 \AA~and 2 m lines for a lower
mass (low luminosity) double detonation model, which has a post-explosion He
mass of 0.077 . In this case, both the He I features we consider are
strong and can provide a clear observational signature of the double-detonation
mechanism.Comment: 12 pages, 11 figures, accepted by A&
Spectral sequences of Type Ia supernovae. I. Connecting normal and sub-luminous SN Ia and the presence of unburned carbon
Type Ia supernovae are generally agreed to arise from thermonuclear
explosions of carbon-oxygen white dwarfs. The actual path to explosion,
however, remains elusive, with numerous plausible parent systems and explosion
mechanisms suggested. Observationally, type Ia supernovae have multiple
subclasses, distinguished by their lightcurves and spectra. This raises the
question whether these reflect that multiple mechanisms occur in nature, or
instead that explosions have a large but continuous range of physical
properties. We revisit the idea that normal and 91bg-like supernovae can be
understood as part of a spectral sequence, in which changes in temperature
dominate. Specifically, we find that a single ejecta structure is sufficient to
provide reasonable fits of both the normal type Ia supernova SN~2011fe and the
91bg-like SN~2005bl, provided that the luminosity and thus temperature of the
ejecta are adjusted appropriately. This suggests that the outer layers of the
ejecta are similar, thus providing some support of a common explosion
mechanism. Our spectral sequence also helps to shed light on the conditions
under which carbon can be detected in pre-maximum SN~Ia spectra -- we find that
emission from iron can "fill in" the carbon trough in cool SN~Ia. This may
indicate that the outer layers of the ejecta of events in which carbon is
detected are relatively metal poor compared to events where carbon is not
detected
Spectral modeling of type II supernovae. I. Dilution factors
We present substantial extensions to the Monte Carlo radiative transfer code
TARDIS to perform spectral synthesis for type II supernovae. By incorporating a
non-LTE ionization and excitation treatment for hydrogen, a full account of
free-free and bound-free processes, a self-consistent determination of the
thermal state and by improving the handling of relativistic effects, the
improved code version includes the necessary physics to perform spectral
synthesis for type II supernovae to high precision as required for the reliable
inference of supernova properties. We demonstrate the capabilities of the
extended version of TARDIS by calculating synthetic spectra for the
prototypical type II supernova SN1999em and by deriving a new and independent
set of dilution factors for the expanding photosphere method. We have
investigated in detail the dependence of the dilution factors on photospheric
properties and, for the first time, on changes in metallicity. We also compare
our results with two previously published sets of dilution factors by Eastman
et al. (1996) and by Dessart & Hillier (2005), and discuss the potential
sources of the discrepancies between studies.Comment: 16 pages, 12 figures, 2 tables, accepted for publication in A&
A reconnaissance of the possible donor stars to the Kepler supernova
The identity of Type Ia supernova progenitors remains a mystery, with various
lines of evidence pointing towards either accretion from a non-degenerate
companion, or the rapid merger of two degenerate stars leading to the
thermonuclear destruction of a white dwarf. In this paper we spectroscopically
scrutinize 24 of the brightest stars residing in the central 38" x 38" of the
SN 1604 (Kepler) supernova remnant to search for a possible surviving companion
star. We can rule out, with high certainty, a red giant companion star - a
progenitor indicated by some models of the supernova remnant. Furthermore, we
find no star that exhibits properties uniquely consistent with those expected
of a donor star down to L>10Lsun. While the distribution of star properties
towards the remnant are consistent with unrelated stars, we identify the most
promising candidates for further astrometric and spectroscopic follow-up. Such
a program would either discover the donor star, or place strong limits on
progenitor systems to luminosities with L<<Lsun.Comment: accepted by Ap
Limits on stable iron in TypeIa supernovae from NIR spectroscopy
We obtained optical and near-infrared spectra of TypeIa supernovae
(SNeIa) at epochs ranging from 224 to 496 days after the explosion. The
spectra show emission lines from forbidden transitions of singly ionised iron
and cobalt atoms. We used non-local thermodynamic equilibrium (NLTE) modelling
of the first and second ionisation stages of iron, nickel, and cobalt to fit
the spectra using a sampling algorithm allowing us to probe a broad parameter
space. We derive velocity shifts, line widths, and abundance ratios for iron
and cobalt. The measured line widths and velocity shifts of the singly ionised
ions suggest a shared emitting region. Our data are fully compatible with
radioactive Ni decay as the origin for cobalt and iron. We compare the
measured abundance ratios of iron and cobalt to theoretical predictions of
various SNIa explosion models. These models include, in addition to
Ni, different amounts of Ni and stable Fe. We can
exclude models that produced only Fe or only Ni in addition to
Ni. If we consider a model that has Ni, Ni, and
Fe then our data imply that these ratios are Fe / Ni
and Ni / Ni .Comment: 10 pages, 7 figures, Accepted for publication in A&
Type Iax SNe as a few-parameter family
We present direct spectroscopic modeling of five Type Iax supernovae (SNe)
with the one dimensional Monte Carlo radiative transfer code TARDIS. The
abundance tomography technique is used to map the chemical structure and
physical properties of the SN atmosphere. Through via fitting of multiple
spectral epochs with self-consistent ejecta models, we can then constrain the
location of some elements within the ejecta. The synthetic spectra of the
best-fit models are able to reproduce the flux continuum and the main
absorption features in the whole sample. We find that the mass fractions of
IGEs and IMEs show a decreasing trend toward the outer regions of the
atmospheres using density profiles similar to those of deflagration models in
the literature. Oxygen is the only element, which could be dominant at higher
velocities. The stratified abundance structure contradicts the well-mixed
chemical profiles predicted by pure deflagration models. Based on the derived
densities and abundances, a template model atmosphere is created for the SN Iax
class and compared to the observed spectra. Free parameters are the scaling of
the density profile, the velocity shift of the abundance template, and the peak
luminosity. The results of this test support the idea that all SNe Iax can be
described by a similar internal structure, which argues for a common origin of
this class of explosions.Comment: 21 pages, 7 tables, 16 figures, accepted by MNRA
Hunting for the progenitor of SN 1006: High resolution spectroscopic search with the FLAMES instrument
Type Ia supernovae play a significant role in the evolution of the Universe
and have a wide range of applications. It is widely believed that these events
are the thermonuclear explosions of carbon-oxygen white dwarfs close to the
Chandrasekhar mass (1.38 M\odot). However, CO white dwarfs are born with masses
much below the Chandrasekhar limit and thus require mass accretion to become
Type Ia supernovae. There are two main scenarios for accretion. First, the
merger of two white dwarfs and, second, a stable mass accretion from a
companion star. According to predictions, this companion star (also referred to
as donor star) survives the explosion and thus should be visible in the center
of Type Ia remnants. In this paper we scrutinize the central stars (79 in
total) of the SN 1006 remnant to search for the surviving donor star as
predicted by this scenario. We find no star consistent with the traditional
accretion scenario in SN1006.Comment: 11 pages, accepted by Ap
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